The present invention relates to apparatus for stacking flexible sheets which consist of paper or the like, and more particularly to improvements in apparatus for converting a stream of partially overlapping sheets into a pile wherein the sheets are accurately stacked on top of each other.
Sheets which are obtained by severing a continuous web of paper or the like at regular intervals are normally assembled into stacks for further processing in printing plants or in other types of establishments. In many instances, the sheets which are separated from the leader of a continuous web by a suitable knife are assembled into a stream of partly overlapping sheets to reduce the speed of forward movement of sheets and to thus facilitate accurate stacking of sheets on top of each other. The formation of a stream of partly overlapping sheets is particularly desirable when the frequency at which the web is severed is very high so that it is necessary to greatly reduce the speed of sheets before they reach a magazine, a platform or an analogous support in or on which the sheets are piled on top of each other. Accurate stacking of sheets is desirable for a number of reasons, e.g., to reduce waste which is a necessary adjunct of secondary treatment (trimming) of stacks wherein the sheets are not in accurate register with each other.
As mentioned above, accurate stacking of sheets can be achieved by reducing the speed of sheets which approach the stacking station. The aforementioned formation of a stream of partially overlapping sheets is an effective procedure to reduce the speed of sheets between the severing and stacking stations. The speed of sheets cannot be reduced at will because, otherwise, the sheets are likely to come to a full stop ahead of the optimum position of accurate overlap with the preceding sheets. The selection of such speed depends, among other factors, on the weight of sheets and the finish of their surfaces.
German Offenlegungschrift No. 1,461,212 discloses a stacking apparatus wherein the lower reaches or stretches of two conveyor belts travel below the bottom wall of a suction chamber at a level above the stacking station. The bottom wall of the suction chamber has an elongated slot which is flanked by the lower reaches of the belts. Thus, when a sheet is fed to the undersides of the lower reaches of the belts, such sheet is attracted by the suction chamber and is moved forwardly by the two lower reaches. The pressure in the interior of the suction chamber is only slightly less than atmospheric pressure, especially when the sheets are readily flexible, because excessive suction would cause the sheet to flex or bulge and to enter the slot between the belts. Moreover, such flexing or bulging of the sheet into the interior of the suction chamber would interfere with orderly transport of the sheet and would prevent the apparatus from stacking successive sheets with a requisite degree of reproducibility. When a sheet reaches its foremost position, it is mechanically stripped off the lower reaches of the belts by a forked separating device which directs the separated sheet onto the topmost sheet of the stack therebelow.
The just described apparatus is not suited for stacking of readily flexible lightweight sheets because the slot in the bottom wall of the suction chamber invariably causes at least some deformation of readily flexible sheets and also because the mechanical separating device often or invariably deforms the leading edges of the sheets during stripping off the lower reaches of the belts. If the pressure in the suction chamber is only slightly less than atmospheric pressure, so that the slotted bottom wall of the suction chamber is unlikely to deface or deform the sheets, the apparatus is incapable of insuring proper transport of each and every sheet all the way into the range of the mechanical separating device.